This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Molecular mechanisms controlling bradyzoite differentiation in the parasite are completely unknown. Dr. Radke has shown previously that Toxoplasma Type I strains are naturally resistant to the artificially induced bradyzoite differentiation and to over-expression of some host cell genes. Dr. Radke's lab uses (1) a complementation strategy to transfer a gene from the Type I RH strain to a Type III strain CTG parasite to identify genes that may render Type I strains resistant to development;and (2) testing histone methylation as one primary mechanism driving induced parasite development when distinct host cell genes are over-expressed in the infected cell. They have transferred a gene from the Type I RH strain to a Type III strain CTG parasite and recovered sequence that maps to ChrVIII beginning at 6300 Kbp and ChrXII at 1200 Kbp. Present work will focus on narrowing the candidate resistance sequences to a single gene that may underlie the molecular mechanisms controlling bradyzoite differentiation and the pathogenesis of chronic infection in the host. Dr. Radke has recently shown that new methylation at lysines 4, 9, and 36 of histone 3 occur in concert with the over-expression of CDA1 in HeLa cells, but whether this is a direct action of CDA1 is unknown. Work to confirm (or not) direct methylation of core histone residues by CDA1 will be completed using site-directed mutagenesis. Mutation of select residues in the SET domain that are able to reduce methylation at lysine 4, 9, 27, or 36 relative to the wild-type protein would suggest CDA1 is able to directly methylate histone residues.